1. Field of the Invention
The invention relates to a process for stabilizing unsaturated organosilicon compounds.
2. Background Art
Organosilicon compounds having unsaturated organic functional groups such as vinyl, acryloyl or methacryloyl groups are widely used as adhesion promoters between inorganic and organic materials, for example in sizes for glass fibers, as crosslinking agents in organic polymers, and as agents for the treatment of fillers.
Preparation processes for such compounds include, for example, the reaction between silanes having SiH bonds and unsaturated (optionally polyunsaturated) organic compounds, generally referred to as “hydrosilylation”, which is catalyzed by metal compounds, and the reaction of haloalkyl-substituted alkoxysilanes with salts of acrylic or methacrylic acid, referred to as “nucleophilic substitution”. Common to all these processes is that they take place exothermically at elevated temperatures, giving rise to the risk of polymerization of the products during the reaction by reaction of the unsaturated organic group, with the result that product is lost and reaction apparatuses used have to be cleaned by complicated procedures. Moreover, the silanes bearing unsaturated organic groups are generally purified by distillation, the thermal energy required for this purpose also giving rise to a considerable risk of polymerization. Finally, there is also the risk of polymerization during storage of these compounds.
Numerous processes for preventing the polymerization of organosilicon compounds carrying unsaturated organic groups include the use of so called free radical polymerization inhibitors. These compounds are used during the preparation, regardless of the preparation route, during the work up by distillation, and during storage of the organosilicon compounds bearing unsaturated groups.
U.S. Pat. No. 5,145,979 describes a mixture of a sterically hindered phenol, an aromatic amine and/or an alkylamine as having a stabilizing effect in the preparation of organosilicon compounds having unsaturated organic groups by hydrosilylation. DE 4 437 667 A1, EP 0 483 479 A1 and EP 0 483 480 A1 describe the use of N,N′-diphenyl-p-phenylenediamine (“DPPA”) for stabilization during the preparation of unsaturated organosilicon compounds by nucleophilic substitution.
EP 1 004 587 A2 discloses use of a combination of two different polymerization inhibitors: a compound from the class consisting of N,N′-disubstituted p-phenylenediamines, and a compound from the class consisting of the 2,6-di-tert-butyl-4-alkylphenols.
EP 708 081 B1, on the other hand, describes the use of N,N′-disubstituted p-quinodiimines, alone or in combination with other compounds having a stabilizing effect, in the preparation of organosilicon compounds bearing unsaturated groups by nucleophilic substitution.
Common to all processes in which a plurality of inhibitors is used is that one of the polymerization inhibitors employed has a very high boiling point and thus stabilizes the bottom product, while another has a volatility similar to that of the product to be distilled and thus stabilizes the gas phase. However, the use of amines such as N,N′-diphenyl-p-phenylenediamine, or sterically hindered phenols containing amino groups such as 2,6-dialkyl4-N,N-dialkyl-aminomethylphenols, leads to the formation of volatile amine impurities under the thermal conditions of the distillation, which result in the distilled products having a yellow color and an unpleasant odor reminiscent of fish.
Common to the processes described are the further disadvantages that relatively large amounts of stabilizing compound(s) have to be used; that these compounds are often very expensive; and that the processes described often, for example when in contact with an oxygen-containing gas mixture, must be considered safety-critical. In addition, in the case of most of the compounds described, in spite of a stabilizing effect exerted on unsaturated organosilicon compounds, there is still a residual risk that the unsaturated organosilicon compounds will be polymerized and hence lost.